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1. Operator: != and <>
- In MySQL
!=and<>consistent functionality, it is recommended sql92 specification is:!=The new specification is recommended:<>
The following query for users whose username is other than "Chen Haha", the following two sentences have the same effect.
SELECT * FROM t_user WHERE username != "陈哈哈";
SELECT * FROM t_user WHERE username <> "陈哈哈";
1. Operators: = and <=>
It is worth mentioning that =, <=>as well as isthe use of the three operators
We all know that is designed to determine whether NULL, and =is used to determine all types of data other than the use of non-NULL. And <=>it is the first two together.
First provide the test table data as follows:
mysql> SELECT * from t_user;
+----+-----------+----------+
| id | username | password |
+----+-----------+----------+
| 1 | 陈哈哈 | abcd1234 |
| 2 | 侨布斯 | 1234 |
| 3 | 提莫 | 1234abcd |
| 4 | aaa | NULL |
| 5 | NULL | aaaa |
+----+-----------+----------+
5 rows in set (0.00 sec)
The results of the following two SQL queries are the same, and data with NULL username is found
mysql> SELECT * from t_user where `username` is null;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 5 | NULL | aaaa |
+----+----------+----------+
1 row in set (0.00 sec)
mysql> SELECT * from t_user where `username` <=> null;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
| 5 | NULL | aaaa |
+----+----------+----------+
1 row in set (0.00 sec)
The results of the following two SQL queries are the same, and the data whose username is'Chen Haha' are found
mysql> SELECT * from t_user where `username` = '陈哈哈';
+----+-----------+----------+
| id | username | password |
+----+-----------+----------+
| 1 | 陈哈哈 | abcd1234 |
+----+-----------+----------+
1 row in set (0.00 sec)
mysql> SELECT * from t_user where `username` <=> '陈哈哈';
+----+-----------+----------+
| id | username | password |
+----+-----------+----------+
| 1 | 陈哈哈 | abcd1234 |
+----+-----------+----------+
1 row in set (0.00 sec)
It can be seen that the <=>operator is equivalent to encapsulating =and is, which can be used to judge non-NULL values or NULL values.
- <=> Only used for MySQL database,
username <=> NULL0 is equivalent to username is NULL,NOT(username <=> NULL)equivalent to username is NOT NULL; - When col1 and col2 two columns with possible NULL values need to be compared for equality, col1 <=> col2 can be used, and the null=null can also be associated.